Level gages are used to determine liquid level in opaque tankage. The simplest and most reliable type is a liquid sight monitor, also known as a direct reading level gage, or sight glass. It comprises a vertical translucent tube, generally constructed of glass or plastic, positioned outside of the tank and connected to the tank so that the liquid level in the tube is the same as the liquid level in the tank. An operator can determine the liquid level in the tank simply by viewing the liquid level in the tube. Direct reading tubular level gages are commonly employed on tanks having a working pressure of less than about 300 psig.
Other types of level gages for tanks generally rely on electronics and/or a float for operability. An advantage of an electronic gage is that the level reading can be near instantaneously transmitted to a remote location for a continuous level readout. The electronic level signal can be used in automated process control. A potential drawback to the use of these types of gages is failure. Electronics and floats can both fail, making it extremely difficult to accurately determine the liquid level in the tank. It is also inherently difficult to verify that the signal produced by these types of gages is indeed representative of liquid level in the tank.
Redundant systems are known. However, there exists need for providing redundancy in an increased cost-effective manner, and for cost effectively retrofitting existing sight glass systems to provide an electrical signal output for remote monitoring and automated control.